Control systems are used to monitor and control inventories of industrial processes. Typically, the control system performs these functions using field devices distributed at key locations in the industrial process and coupled to the control circuitry in the control center by a process control loop. The term field device refers to any device that performs a function in a distributed control or process monitoring system, including all devices used in the measurement, control, and monitoring of industrial processes. Industrial process, as used herein, refers, for example, to an oil refinery, a steel or other metal works, a chemical production facility, or an electrical power generation station.
Some field devices may include a transducer. As used herein, a transducer is understood to mean either a device that generates an output signal based on a physical input or that generates a physical output based on an input signal. Typically, a transducer transforms an input into an output having a different form. Transducer may include, for example, a pressure sensor, thermistor, thermocouple, strain gauge, flow meter, pH meter, positioner, actuator, solenoid, stepper motor, relay, and indicator light.
Typically, each field device also includes communication circuitry that is used for communicating with a process control center, or other circuitry, over a process control loop. In some installations, the process control loop is also used to deliver a regulated current and/or voltage to the field device for powering the field device. The process control loop also carries data either in analog or digital format.
Traditionally, analog field devices have been connected to the control center by two-wire process control current loops, with each device connected to the control center by a single two-wire control loop. Typically, a voltage differential is maintained between the two wires of the process control loop generally within a range of voltages from 12-45 volts for analog mode and 9-50 volts for digital mode. Some analog field devices transmit a signal to the control center by modulating the current running through the current loop to a current proportional to the sensed process variable. Other analog field devices can perform an action under the control of the control center by controlling the magnitude of the current through the loop. In addition to, or in the alternative, the process control loop can carry digital signals used for communication with field devices. Digital communication allows a much larger degree of communication than analog communication. Field devices that communicate digitally can respond to and communicate selectively with the control center and/or other field devices. Further, such devices can provide additional signaling such as diagnostics and/or alarms.
In some installations, wireless technologies have begun to be used to communicate with field devices. Wireless operation simplifies field device wiring and setup. Wireless installations are currently used in which the field device is manufactured to include an internal battery, potentially charged by a solar cell, or other technique to obtain power without any sort of wired connection. Problems exist in using an internal battery as the energy demands of wireless devices may vary greatly depending on numerous factors such as the device reporting rate, device elements, et cetera, so that the battery may become exhausted unpredictably. When the battery is exhausted, replacement of the battery usually requires a technician specially trained for that task. The battery may be physically difficult to access for replacement, for example, due to elevation, due to being located within a nest of pipes or equipment, due to being placed in a hazardous location.
It is known to transmit power to a field device using non-radiative fields. For example, magnetic induction may be used to transfer energy from a primary coil to a secondary coil without a direct electrical connection. See U.S. Patent App. Pub. No. 2012/0305096 to Haller. Inductive chargers, such as those found commonly in electric toothbrushes, operate on this same principle. However, for these systems to operate efficiently, the primary coil (source) and secondary coil (device) must be located in close proximity and carefully positioned with respect to one another. Thus, the magnetic coupling between the source and device coils must be large for proper operation. Furthermore, one primary coil transfers energy to one secondary coil.
Accordingly, there is a need for improved apparatus as well as related methods for control of industrial processes.